linux-kernel - Re: [PATCH 01/23] sched: Provide sched_set

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Date:   Wed, 22 Apr 2020 15:26:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de,
        rostedt@...dmis.org, qais.yousef@....com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, airlied@...hat.com,
        alexander.deucher@....com, awalls@...metrocast.net,
        axboe@...nel.dk, broonie@...nel.org, daniel.lezcano@...aro.org,
        gregkh@...uxfoundation.org, hannes@...xchg.org,
        herbert@...dor.apana.org.au, hverkuil@...all.nl,
        john.stultz@...aro.org, nico@...xnic.net,
        rafael.j.wysocki@...el.com, rmk+kernel@....linux.org.uk,
        sudeep.holla@....com, ulf.hansson@...aro.org,
        wim@...ux-watchdog.org
Subject: Re: [PATCH 01/23] sched: Provide sched_set_fifo()

On Wed, Apr 22, 2020 at 06:11:38AM -0700, Paul E. McKenney wrote:
> On Wed, Apr 22, 2020 at 01:27:20PM +0200, Peter Zijlstra wrote:
> > SCHED_FIFO (or any static priority scheduler) is a broken scheduler
> > model; it is fundamentally incapable of resource management, the one
> > thing an OS is actually supposed to do.
> > 
> > It is impossible to compose static priority workloads. One cannot take
> > two well designed and functional static priority workloads and mash
> > them together and still expect them to work.
> > 
> > Therefore it doesn't make sense to expose the priority field; the
> > kernel is fundamentally incapable of setting a sensible value, it
> > needs systems knowledge that it doesn't have.
> > 
> > Take away sched_setschedule() / sched_setattr() from modules and
> > replace them with:
> > 
> >   - sched_set_fifo(p); create a FIFO task (at prio 50)
> >   - sched_set_fifo_low(p); create a task higher than NORMAL,
> > 	which ends up being a FIFO task at prio 1.
> >   - sched_set_normal(p, nice); (re)set the task to normal
> > 
> > This stops the proliferation of randomly chosen, and irrelevant, FIFO
> > priorities that dont't really mean anything anyway.
> > 
> > The system administrator/integrator, whoever has insight into the
> > actual system design and requirements (userspace) can set-up
> > appropriate priorities if and when needed.
> 
> The sched_setscheduler_nocheck() calls in rcu_spawn_gp_kthread(),
> rcu_cpu_kthread_setup(), and rcu_spawn_one_boost_kthread() all stay as
> is because they all use the rcutree.kthread_prio boot parameter, which is
> set at boot time by the system administrator (or {who,what}ever, correct?

Correct, also they are not modular afaict, so they escaped the dance ;-)